Second Species (two notes against one note)
Most of the rules in second species remain the same as in first species. However, the student now adds two notes for each note of the cantus firmus so every other note is an offbeat onto which the student may introduce dissonances.

This is because the two dissonant notes do not sound at the same time as they would in first species. There are still strict conditions, however, to avoid the dissonances standing out too much. Dissonances may appear:

only on the second minim of each bar

if they are approached by step (i.e. there must be an interval of a tone or a semitone between the dissonance and the previous note)

if they are left by step in the same direction in which they are approached.

Another difference between this and first species is that, because there are two notes per bar parallel fifths and octaves are allowed between the first minim of one bar and the first minim of the next. Again, there are rules that govern this:

the passing note in between the two perfect fifths must leap away a perfect fourth from the first minim

the passing note must also move in contrary motion against the cantus firmus